The present invention relates to an apparatus for the treatment of water to remove therefrom colloidal and suspended matter by flocculation and/or crystalline precipitation, followed by lamella settling to achieve separation of sludges formed by settling, concentrating or thickening, and recirculation or recycling of a portion of the sludge after concentration thereof.
The present invention is directed to such an apparatus applicable to the treatment of all types of water, for example surface water charged with colloidal and suspended matter, waste water, and particularly advantageously to water loaded with mineral soils that can be precipitated in the form of crystalline sludge by a reagent, for example such as lime.
In French Pat. No. 76.08442 there is disclosed an apparatus for the treatment of water by recirculation, precipitation and separation of sludges formed. The apparatus includes in the same enclosure two zones, i.e. a reaction zone, for example for coagulation, flocculation or crystalline precipitation, and another zone for lamella settling. The two zones are in communication along the width of the apparatus and allow an internal recirculation or recycling of a portion of the sludge formed.
A number of other devices have been known, for example from French Pat. No. 76.37544, including a flocculator and a lamella settling tank with recirculation or recycling of sludge from the settling tank to the flocculator by means of a hydroejector.
However, very often the factor limiting capacity of these types of devices is not the settling velocity, but the volume of sludge produced during treatment. Moreover, the correct operation of these types of devices is conditioned upon the maintenance in the reaction/flocculation zone of an appropriate concentration of sludge supplied by the recirculation or recycling flow. The attainment of such a concentration become problematic beyond a particular flow rate of the water to be treated corresponding to a settling velocity which is lower than that theoretically possible in the lamella settling zone.
Indeed, to each increase of the settling velocity and therefore of the flow rate of the water being treated in an apparatus of particular dimensions, there corresponds a proportional increase of sludge production. Since the volume available below the modules of the lamella settling tank for the storage of such sludges cannot vary, it follows that it is impossible, without risking clogging of the apparatus, to comply with the requirement of an adequate detention time below the lamella modules to ensure thickening of the sludge to obtain, on the one hand, the proper concentration of sludge in the recycling flow, and on the other hand, the maximum possible concentration in the flow of sludge withdrawn from the apparatus.
Deepening or increasing the height of the apparatus in order to provide a sufficient volume below the lamella modules for storage and thickening of the sludge does not solve the problem, since the capacity of the apparatus then continues to be limited by the maximum possible value of the horizontal circulation flow which, in order to maintain the required sludge concentration in the flocculation and reaction tank, must be adapted to the flow rate of the volume of water being treated. It has been determined that, starting from a particular value of this recirculation flow corresponding to a treatment flow rate and thus to a settling velocity well below that allowed by the presence of the lamella modules, turbulent flow results below the lamella modules. This leads to an unacceptable deterioration of the quality of the treated water.